This study is done in conjunction with a trial, conducted at Columbia University College of Physicians and Surgeons in New York and the Royal Manchester Children's Hospital in England, to examine the effectiveness of a new drug called OGT 918 for treating Niemann-Pick Type C (NPC) disease. Patients with this genetic disorder do not transport lipids (fatty substances) in their cells, resulting in problems of the liver, spleen and brain. An early sign of NPC is a reduced ability to move the eyes rapidly up and down or from side to side. These voluntary eye movements are called saccades. Patients in the OGT 918 trial who participate in this sub-study will have their saccadic eye movements measured to see if improvement occurs with OGT 918 treatment.